In any electrical or electronic system, power considerations are generally a factor in circuit and system design, and line drivers, including Digital subscriber Line (DSL) drivers, are no exception. A driver typically includes an amplifier stage, having a gain (which may have a value greater than, equal to or less than one), preceding an output stage in a transceiver. Line drivers are used, among other things, to drive or compel a signal (e.g., an analog signal) through a transmission medium. DSL drivers are used to drive signals down a transmission medium such as a twisted pair telephone wire. For a general discussion of DSL, please see “DSL: Simulation Techniques and Standards Development for Digital Subscriber Line Systems,” by Dr. Walter Y. Chen, MacMillian Technical Publishing, 1998, which is hereby incorporated by reference in its entirety.
The power necessary to drive a signal down a transmission medium may vary depending on the line characteristics thereof. For example, given a plurality of telephone wires connected to a central office providing DSL service, each of the plurality of the twisted pair telephone wires generally exhibits different line characteristics thereby necessitating varying power prerequisites to transmit a signal. These needs may be a function of various related or disparate factors, such as a length of the transmission medium, electromagnetic shielding of the transmission medium, and so on.
Another design consideration that should be accommodated for is that the line driver should be designed with a sufficient amount of headroom. Headroom may be generally defined as a design parameter that allows a wider dynamic range associated with a driver's output than is normally associated with the typical root-mean-square average value of the driver's typical output signal.
This allocation of the dynamic range for the line driver may, therefore, accommodate for the transmission of certain bursts of signals with a significantly higher voltage amplitude when compared to the typical root-mean-square average of the signal. In other words, designing for the headroom of a signal may be necessary to accommodate a bursty output signal so that the driver is not forced into distortion.
However, designing for the desired headroom of a driver system, as well as compensating for various non-ideal characteristics of the transmission medium or path, may place even higher power considerations on the driver system. The inefficiencies associated with these considerations may necessitate that the voltage rails in connection with the amplifier stage of the line driver have a wider range, which in turn creates more extreme thermal characteristics, which may in turn lead to a lower density for the drivers than would otherwise be possible.
To combat these above and other considerations and inefficiencies, there have been attempts in the prior art to achieve more efficient amplifier or power driver systems. For instance, driver systems such as the AD8016, by Analog Devices, Incorporated of Norwood, Mass. and the LT1795 by Linear Technologies Corporation of Milpitas, Calif. allow for an adjustment of the bias current in the line driver to control quiescent consumption.
Another attempt to achieve a more efficient amplifier or driver system is disclosed in the U.S. Pat. No. 3,961,280, by Sampei, entitled “Amplifier Circuit Having Power Supply Voltage Responsive to Amplitude of Input Signal,” issued on Jun. 1, 1976, which is hereby incorporated by reference in its entirety. In Sampei, a class of amplifiers, designated as class ‘G’ amplifiers, are disclosed. The amplifier disclosed by Sampei changes the power supply voltages in accordance with the magnitude of an input signal.
Problems persist, however, in association with these various systems and approaches. A limitation of the AD0816 and the LT1795 driver systems from Analog Devices, Incorporated and Linear Technologies Corporation, respectively, is that the devices only control the bias current. It does not control the dominant, dynamic power consumption of the driver. In the case of the amplifier disclosed by Sampei, which is associated with the class ‘G’ amplifiers, there are the problems of high circuit complexity, poor linearity and the need for multiple power supplies, each power supply correlating to given applied power level.
Accordingly, what is needed in the art is a line driver that may adaptively modify the power selection capability associated with transmitting a signal that overcomes the deficiencies in the prior art.